1. Technical Field of the Invention
The invention relates to an optical pickup device, and more particularly to an optical pickup device suitable for use in irradiating a recording medium having plural laminated recording layers with laser light.
2. Disclosure of Related Art
In recent years, as the capacity of an optical disc has been increased, an optical disc having an increased number of recording layers has been developed. Laminating recording layers in a disc enables to considerably increase the data capacity of the disc. In the case where recording layers are laminated, generally, two recording layers are laminated on one side of a disc. Recently, however, laminating three or more recording layers on one side of a disc has been put into practice to further increase the capacity of the disc. Thus, the capacity of a disc can be increased by increasing the number of recording layers to be laminated. However, as the number of recording layers to be laminated is increased, the distance between the recording layers is decreased, and signal deterioration resulting from an interlayer crosstalk is increased.
As the number of recording layers to be laminated is increased, reflection light from a recording layer (a targeted recording layer) to be recorded/reproduced is reduced. As a result, if unwanted reflection light (stray light) is entered into a photodetector from a recording layer on or under the targeted recording layer, a detection signal may be deteriorated, which may adversely affect focus servo control and tracking servo control. In view of this, in the case where a large number of recording layers are laminated, it is necessary to properly remove stray light, and stabilize a signal from a photodetector.
Japanese Unexamined Patent Publication No. 2009-211770 (corresponding to U.S. Patent Application Publication No. US2009/0225645 A1) discloses a novel arrangement of an optical pickup device operable to properly remove stray light, in the case where a large number of recording layers are formed. With this arrangement, it is possible to form an area where only signal light exists, on a light receiving surface of a photodetector. By disposing a sensor of the photodetector in the above area, it is possible to suppress an influence on a detection signal resulting from stray light.
In the above optical pickup device, an area onto which signal light is irradiated, and an area onto which stray light is irradiated are adjacent to each other. As a result, even if a sensor is disposed in an area where only signal light exists, a part of stray light may be entered into the sensor, which may degrade the precision of a detection signal. Further, in the above optical pickup device, if a disc having a small distance between adjacent recording layers is used, it is difficult to determine an S-shaped curve in focus servo control, which makes it difficult to determine a target recording layer.